You are provided with a beaker, a thermometer, a stirrer Stopwatch/clock, measuring cylinder, table salt, water, and other necessary materials. i. You Measu...
Question 1 Report
You are provided with a beaker, a thermometer, a stirrer Stopwatch/clock, measuring cylinder, table salt, water, and other necessary materials.
i. You Measure \(200\text{cm}^{3}\) of water into the beaker.
ii. Heat the water until it boils steadily for about 2 minutes.
iii. Read and record the boiling point \(b_{0}\).
iv. Add table salt of mass \(M = 10.0\text{ g}\) to the boiling water and stir continuously until another boiling point \(b_{1}\) is attained.
v. Read and record \(b_{i}\).
vi. Evaluate \(\theta_{i} = (b_{i} - b_{0})\)
vii. Using the same mixture, repeat the procedure four more times by adding 10.0 g of salt each time to give the cumulative mass \(M_{i}\) of salt as 20 g, 30g, 40g, and 50g.
viii. In each case allow the mixture to boil steadily for at least 2 minutes then read and record the boiling point b.
ix. Tabulate your readings.
x. Plot a graph with \(M_{i}\) on the vertical axis and \(\theta_{1}\) on the horizontal axis.
xi. Determine the slope, s, of the graph.
xii. State two precautions taken to ensure accurate results.
(b) i. Define the boiling point of a liquid.
ii. What effects do impurities have on the boiling point of a liquid?
Answer Details
Effect of dissolved salt on the boiling point of water
The boiling point of the water before salt was added was:
\[b_0=102.0^{\circ}\mathrm{C}\]
| S/N | Cumulative mass, \(M_i\) / g | Boiling point, \(b_i\) / \(^\circ\mathrm{C}\) | \(\theta_i=b_i-b_0\) / \(^\circ\mathrm{C}\) |
|---|---|---|---|
| 1 | 10.0 | 104.0 | \(104.0-102.0=2.0\) |
| 2 | 20.0 | 106.0 | \(106.0-102.0=4.0\) |
| 3 | 30.0 | 108.0 | \(108.0-102.0=6.0\) |
| 4 | 40.0 | 110.0 | \(110.0-102.0=8.0\) |
| 5 | 50.0 | 112.0 | \(112.0-102.0=10.0\) |
Graph of cumulative mass of salt against rise in boiling point
Using two widely separated points on the line, \((\theta_1,M_1)=(2.0,10.0)\) and \((\theta_2,M_2)=(10.0,50.0)\):
\[s=\frac{\Delta M}{\Delta\theta}=\frac{50.0-10.0}{10.0-2.0}=\frac{40.0}{8.0}=5.0\ \mathrm{g\,^{\circ}C^{-1}}.\]
Slope, \(s=5.0\ \mathrm{g\,^{\circ}C^{-1}}\).
Precautions
- Stir the solution gently and continuously so that its temperature is uniform throughout.
- Take each thermometer reading only after steady boiling has continued for at least 2 minutes, reading the scale at eye level to avoid parallax error.
(b)(i) Boiling point
The boiling point of a liquid is the temperature at which its saturated vapour pressure equals the external atmospheric pressure, so that vaporisation occurs throughout the liquid.
(b)(ii) Effect of impurities
Dissolved impurities raise the boiling point of a liquid. Thus, as more salt is dissolved in the water, a higher temperature is required for it to boil.
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